From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 5/6] cgroup: implement cgroup v2 thread support
Date: Mon, 17 Jul 2017 10:26:09 -0400
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To: Peter Zijlstra <peterz@infradead.org>
Cc: lizefan@huawei.com, hannes@cmpxchg.org, mingo@redhat.com, longman@redhat.com, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, pjt@google.com, luto@amacapital.net, efault@gmx.de, torvalds@linux-foundation.org, guro@fb.com

Hello, Peter.

On Mon, Jul 17, 2017 at 04:14:09PM +0200, Peter Zijlstra wrote:
> AFAICT this is not in fact what I suggested... :/

Heh, sorry about misattributing that.  I was mostly referring to the
overall idea of marking each cgroup domain or threaded rather than
subtree.

> My proposal did not have that invalid state. It would simply refuse to
> change the type from thread to domain in the case where the parent is
> not a domain.
> 
> Also, my proposal maintained the normal property inheritance rules. A
> child cgroup's creation 'type' would be that of its parent and not
> always be 'domain'.

But aren't both of the above get weird when the parent can host both
domain and threaded children?

	 R
       / 
      A(D)

If you create another child B under R, it's naturally gonna be a
domain.  Let's say you turn that to threaded.

	 R
       /   \
     A(D) B(T)

And now try to create another child C, should that be a domain or
threaded?

If we only inherit from the second level on, which is in itself
already confusing, that still leads to invalid configs for non-root
thread roots.

I don't think whether we fail the transition or put the cgroup in an
invalid state is all that material.  The simpler the better.

> Let me read more (and more careful) to see if there's other things.

Sure thing.

Thanks!

-- 
tejun